In optical data recording, an optical source, typically a laser or laser diode, generates an incident write signal in the form of a radiation beam. The beam is applied to an optical medium to record data thereon as optically-detectable marks. To facilitate proper application of the beam to the medium, certain types of media, such as compact disk recordable (CD-R) media, include a preformed spiral tracking structure typically referred to as a groove or pregroove. The spiral groove may be "wobbled" in a radial direction of the disk about an average groove centerline in order to provide rotational velocity and beam position information in a well-known manner. The dimensions and shape of the groove can vary depending upon the medium, but an exemplary groove width is on the order of 0.4 .mu.m in the radial direction, with adjacent grooves in the spiral separated by about 1.6 .mu.m in the radial direction. In CD-R media, the Groove corresponds to a data track on which marks are recorded.
The quality of recorded data in many optical recording systems is generally very sensitive to the cross-track position of the recording spot on the medium. In a CD-R system, for example, data quality rapidly deteriorates when the recording spot deviates from the centerline of the disk groove. A commonly-used technique for maintaining a recording spot on-track is referred to as push-pull tracking. The technique involves deriving a push-pull tracking signal from an interference pattern caused by the interaction of the recording spot with the Groove or other suitable tracking structure on the recording medium. A tracking servo adjusts the position of the recording spot to keep the push-pull signal at a predetermined optimum value Generally referred to as a "tracking offset" or an on-track value. The tracking offset is intended to compensate for static errors such as detector misalignment and optical axis tilt. Prior art techniques typically determine the tracking offset during a calibration period before data recording. One common approach is to make a series of trial recordings.
U.S. Pat. No. 5,440,534, assigned to the present assignee and incorporated by reference herein, discloses a technique for determining the tracking offset in an optical recorder by correlating a mark formation signal generated during one or more trial recordings with a corresponding push-pull tracking signal. This technique recognizes and makes use of the fact that certain characteristics of the mark formation signal vary with cross-track position of the recording spot, thereby providing a number of advantages over other techniques. For example, the tracking offset can be determined in a single recording pass, as compared to separate record and read passes required by two-pass techniques. In addition, the resulting tracking offset is optimized for recording rather than for reading.
These and other push-pull tracking techniques are generally "single spot" techniques in that the spot that records and reads data also generates the tracking signal. This reduces system cost and complexity when compared with multi-spot tracking techniques. However, push-pull tracking also suffers from a number of problems. For example, the tracking offset can vary with conditions such as media tilt, lens decenter, optical spot aberrations, and groove asymmetry. These conditions can change from disk to disk and from point to point on a given disk. The tracking offset determined during a calibration period may therefore be unable to keep the recording beam sufficiently on-track in the presence of the changing conditions that can arise during actual data recording.
As is apparent from the above, a need exists for a dynamic tracking technique in which a recording beam is maintained on-track using tracking offset information generated during actual data recording.